Pressure swing adsorption (PSA) provides an efficient and economical means for separating a multi-component gas stream containing at least two gases having different adsorption characteristics. The more strongly adsorbable gas can be an impurity which is removed from the less strongly adsorbable gas which is taken off as product; or, the more strongly adsorbable gas can be the desired product, which is separated from the less strongly adsorbable gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce a purified (99+%) hydrogen stream for a hydrocracking or other catalytic process where these impurities could adversely affect the catalyst or the reaction. On the other hand, it may be desired to recover more strongly adsorbable gases, such as ethylene, from a feedstream to produce an ethylene-rich product.
In pressure swing adsorption, a multi component gas is typically fed to at least one of a plurality of adsorption zones at an elevated pressure effective to adsorb at least one component, while at least one other component passes through. At a defined time, the feedstream to the adsorber is terminated and the adsorption zone is depressurized by one or more cocurrent depressurization steps wherein pressure is reduced to a defined level which permits the separated, less strongly adsorbed component or components remaining in the adsorption zone to be drawn off without significant concentration of the more strongly adsorbed components. Then, the adsorption zone is depressurized by a countercurrent depressurization step wherein the pressure on the adsorption zone is further reduced by withdrawing desorbed gas countercurrently to the direction of feedstream. Finally, the adsorption zone is purged and repressurized. The final stage of repressurization is typically with product gas and is often referred to as product repressurization.
In multi-zone systems there are typically additional steps, and those noted above may be done in stages. U.S. Pat. Nos. 3,176,444 issued to Kiyonaga, 3,986,849 issued to Fuderer et al., and 3,430,418 and 3,703,068 both issued to Wagner, among others, describe multi-zone, adiabatic pressure swing adsorption systems employing both cocurrent and countercurrent depressurization, and the disclosures of these patents are incorporated by reference in their entireties.
Various classes of adsorbents are known to be suitable for use in PSA systems, the selection of which is dependent upon the feedstream components and other factor generally known to those skilled in the art. In general, suitable absorbents include molecular sieves, silica gel, activated carbon and activated alumina. When PSA processes are used to purify hydrogen-containing streams, the hydrogen is essentially not absorbed on the adsorbent. However, when purifying methane-containing streams, methane is often absorbed on the adsorbent along with the impurity. The phenomenon is known in the PSA art as coadsorption.
The coadsorption of methane often causes a temperature rise in the adsorption zone due to the exothermic heat of adsorption which can be substantial, e.g., 40.degree. F. or more. The degree of temperature rise depends, in part, upon the amount of methane present and the particular absorbent employed. As is well known in the art, such a temperature rise can be undesirable since the equilibrium loading of many adsorbates, e.g., ethane, is reduced at higher temperatures. Hence it would be desirable to reduce such temperature rises during adsorption. Similarly, during PSA regeneration, the desorption of the coadsorbed methane along with the impurity often causes a temperature decrease of about the same magnitude as the previously mentioned temperature rise due to the endothermic heat of desorption. Also, as known in the art, such a temperature decrease during desorption can be undesirable since as noted above, the equilibrium loading of many adsorbates, e.g., ethane, is reduced at higher temperatures. Accordingly it would be desirable to reduce the temperature decreases often observed during desorption. However, as can be discerned from the above, the actual thermal behavior of the PSA system runs counter to what is desired. Nevertheless, a variety of processes have been proposed for purifying methane that utilize pressure swing adsorption.
U.S. Pat. No. 3,594,983 issued to Yearout et al., discloses a process for the removal of ethane and other hydrocarbons in a natural gas feedstream, i.e., methane, using a process that employes both pressure swing and thermal swing adsorption. The patentees recognized that zeolitic molecular sieves, e.g., 13X and 4A, were suitable for use in the process due to their high affinity for the impurity components. The patentees' apparent solution to the above-described thermal problem associated with methane coadsorption was to incorporate the thermal swing step into the adsorption process. That is, an adsorption zone that had been previously subjected to pressure reduction, i.e., PSA, was thereafter heated to desorb remaining impurities not removed by pressure swing.
Although not directly related to the separation of ethane from methane containing streams, U.S. Pat. No. 4,775,396 issued to Rastelli et al. discloses a PSA process for the bulk separation of CO.sub.2 from methane, e.g., landfill gas. The patent discloses that for purification processes, CO.sub.2 can be effectively removed from gas mixtures containing same using the calcium ion-exchanged form of zeolite A, but because of the strong affinity between the sorbent and adsorbate, thermal energy is required for effective desorption of the CO.sub.2. However, for the bulk removal of CO.sub.2 from methane, the patent discloses that PSA can be effective when using faujasite type of zeolitic aluminosilicate containing at least 20 equivalent percent of at least one cation species selected from the group consisting of zinc, rare earth, hydrogen and ammonium and containing not more than 80 equivalent percent of alkali metal or alkaline earth metal cations.
Japanese Patent No. 1039163, issued Mar. 31, 1981 to Union Carbide Corp., discloses a process for the purification of methane by the removal of ethane from a methane-containing feedstream that does not require the use of a thermal swing regeneration step. The patent discloses a PSA process that employs the use of silica gel as the adsorbent. The patent discloses that the silica gel adsorbent provides (1) high differential loading for all impurities to be removed from the product methane, (2) good enrichment of impurities in the waste gas, and (3) ease of cleaning of the bed with low pressure purge gas. It is further stated that high differential loadings permit relatively small adsorption zones which are low in cost and which reduce frequency of desorption, and hence reduce the product loss associated therewith. Enrichment of impurities in the waste gas reflects the degree of separation achievable in the process and is important in order to reject the impurities with minimum loss of product component. Ease of cleaning (or desorption) permits a high purity methane product to be obtained with an economically small quantity of purge gas. The above-identified characteristics stress the importantance of achieving both high purity and high recovery of methane.
As noted in the above-identified Japanese patent, it is desirable to provide both high methane recoveries, i.e., minimum loss of product component, and high purity, i.e., low ethane content. Generally, however, there is an inverse relationship between purity and recovery and as such, processes have been operated to provide a high purity product at low recoveries or a low purity product at high recoveries. For example, it is not uncommon to obtain less than 50% methane recovery when purities are maintained at about 300 ppm ethane or less. Even at about 1000 ppm ethane in the product methane, typical processes may only achieve about 55% methane recovery. Accordingly, processes for the purification of methane are sought which can provide a high purity product at higher recoveries than heretofore possible.